1. Field of the Invention
The present invention relates to a power switching device which switches between power supplied from a main power supply and power supplied from a supplementary power supply to supply the selected power to a main circuit.
2. Description of the Related Art
Any instantaneous power interruptions are not permitted in facilities, such as fire alarm systems. The control panel of such a facility is equipped with a power switching device. The power switching device continuously feeds reliable power to a main circuit in the control panel by selecting between power from a main power supply and power from a supplementary power supply. FIG. 5 is a block diagram of a conventional power switching device. As shown, the power switching device is housed in the control panel, and switches between power supplied from a main power supply ME (utility service line, for example, a commercial 100-volt power line) external to the control panel and power supplied from a supplementary power supply SE (of 24 VDC, for example) arranged in the control panel.
The conventional power switching device includes an NV (No-Voltage) relay 101 as a switching element, a control unit 102 for controlling the NV relay 101, and a capacitor C100. The control unit 102 includes a voltage detecting unit 121 for detecting the voltage of the power supplied by either the supplementary power supply SE or the main power supply ME, a NOT gate 122, namely, an inverter, for inverting an output signal of the voltage detecting unit 121, and a driver 123 for driving the NV relay 101 in response to an output of the NOT gate 122.
In the power switching device thus constructed, the NV relay 101 is connected to the supplementary power supply SE at its initial state prior to the application of power to the control panel. In the power-on operation of the supplementary power supply and the main power supply, the power supplied by the supplementary power supply is initially supplied to a main circuit MS. When the voltage detecting unit 121 detects that a voltage value of the power supplied by the main power supply ME rises above a predetermined value, the voltage detecting unit 121 outputs a signal indicating a LOW state. The LOW-state signal is inverted by the NOT gate 122 into a HIGH-state signal, which is then fed to the driver 123. In response to the HIGH-state signal, a transistor in the driver 123 is driven, thereby feeding a voltage to the NV relay 101. The NV relay 101 is switched to the side of the main power supply ME. In this way, the power from the main power supply ME is fed to the main circuit MS. This switching operation is performed at the power-on operation because the power supplied by the main power supply is not stable immediately subsequent to the power on. The supplementary power supply thus needs to feed power until the main power supply becomes stabilized.
When the voltage detecting unit 121 detects that the voltage value of the power supplied from the main power supply ME drops below the predetermined value as a result of an interruption of the power from the main power supply ME in the above state, the voltage detecting unit 121 outputs a HIGH-state signal. The NOT gate 122 inverts the HIGH-state signal into a LOW-state signal, which is then fed to the driver 123. With the driver 123 receiving the LOW-state signal, the transistor stops conducting, thereby suspending the voltage application to the NV relay 101. The NV relay 101 is connected back to the supplementary power supply SE. The power from the supplementary power supply is then fed to the main circuit MS.
The conventional power switching device performs the above switching sequence, thereby reliably feeding power to the main circuit MS.
The operation speed of the NV relay 101 is slow. To avoid a power interruption during the slow power switching operation, power stored in the capacitor C100 is used to compensate for a lack of power during the power switching operation. Specifically, the capacitor C100 is charged with the power from the main power supply ME or the supplementary power supply SE, and the capacitor C100 feeds power to the main circuit MS for a duration of several milliseconds, from the moment the NV relay 101 is driven to the completion of the switching operation, in the switching operation. Because of the need for compensating for the power during the switching operation of the slow-speed NV relay 101, a high-capacitance capacitor as high as several tens of thousands micro farads is employed for the capacitor C100.
In the conventional power switching device, the capacitor C is used to preclude a power interruption during the power switching operation. If the switching operation is carried out before the capacitor C is sufficiently charged, the capacitor C has not enough power to discharge. The use of the capacitor C is not a good instantaneous remedial step. The high-capacitance capacitor of several tens of thousands microfarads is expensive, thereby leading to a cost increase of the entire power switching device. Such a capacitor, in need of ample space for installation, presents difficulty in the miniaturization effort of the entire power switching device.